Catalytic combustion systems are well known in gas turbine applications to reduce the creation of pollutants, such as NOx, in the combustion process. One catalytic combustion technique known as the rich catalytic, lean burn (RCL) combustion process includes mixing fuel with a first portion of compressed air to form a rich fuel mixture. The rich fuel mixture is passed over a catalytic surface and partially oxidized, or combusted, by catalytic action, increasing mixture temperature. Activation of the catalytic surface is first achieved when the temperature of the rich fuel mixture is elevated to a temperature at which the catalytic surface becomes active. Typically, compression raises the temperature of the air mixed with the fuel to form a rich fuel mixture having a temperature sufficiently high to activate the catalytic surface. After passing over the catalytic surface, the resulting partially oxidized hot rich fuel mixture is then mixed with a second portion of compressed air in a downstream combustion zone to produce a heated lean combustion mixture for completing the combustion process. Catalytic combustion reactions may produce less NOx and other pollutants, such as carbon monoxide and hydrocarbons, than pollutants produced by homogenous combustion.
U.S. Pat. No. 6,174,159 describes a catalytic combustion method and apparatus for a gas turbine utilizing a backside cooled design. Multiple cooling conduits, such as tubes, are coated on the outside diameter with a catalytic material and are supported in a catalytic reactor. A first portion of a fuel/air mixture, such as 15% by volume of the fuel/air mixture, is passed over the catalyst coated cooling conduits and is catalytically combusted, while simultaneously, a second portion of the fuel/air, such as 85% by volume of the fuel/air mixture, enters the multiple cooling conduits and cools the catalyst. The exothermally catalyzed fluid then exits the catalytic combustion zone and is mixed with the cooling fluid in a downstream post catalytic combustion zone defined by a combustor liner, creating a heated, combustible mixture.
Integrated gasification combined cycle (IGCC) power plants are known to produce synthesis gas, or syngas, from carbon-containing sources such as coal, Biomass and other sources. The syngas is then used to fuel, using a conventional diffusion flame process, a combustor of a gas turbine engine connected to a generator for producing electrical power. In IGCC power plants, the gas turbine is typically required to be capable of being operated on a back-up fuel source, such as natural gas, for example, during startup and periods when syngas is unavailable.
In a conventional IGCC process, an air separation unit (ASU) is used to provide oxygen for a gasifier in a separation process that also produces compressed nitrogen as a byproduct. Typically, the nitrogen produced during the separation process is returned to the combustor. In conventional diffusion flame combustors used in an IGCC power plant, the syngas needs to be diluted to reduce a peak syngas flame temperature to achieve acceptable NOx emissions. Dilution is typically achieved with the injection of ASU produced nitrogen into the syngas provided to the combustor. In addition, injection of nitrogen into the syngas may be needed to satisfy the turbine compressor and expander mass flow requirements.